Field of the Invention
The present invention relates to a microwave plasma treatment apparatus, and more particularly to a microwave plasma treatment apparatus wherein the electron cyclotron resonance (ECR) phenomenon is utilized to form a plasma which is used to carry out a surface treatment of a substrate or the like, examples of which treatment include etching thereof and thin film deposition or formation thereon.
As a conventional microwave plasma treatment apparatus of the type as described above, there is known, for example, an apparatus disclosed in Japanese Patent Laid-Open No. 155,535/1981.
Prior to the description of the present invention, a brief description will first be made of a microwave plasma treatment technique disclosed in the above-mentioned patent literature.
FIG. 1 is a schematic partial front cross-sectional view of the above-mentioned conventional apparatus, wherein the essential constituent parts thereof are illustrated.
In FIG. 1, numeral 10 refers to a cylindrical plasma formation chamber, while numeral 12 refers to an object treatment chamber provided adjacent to the plasma formation chamber 10 in the direction of the central axis O thereof. The plasma formation chamber 10 is usually provided, at the end portion thereof opposite to the object treatment chamber 12 in the direction of the above-mentioned central axis O, with a microwave introducing window 14 made of an insulating material such as quartz glass or a ceramic material, through which microwaves of, for example, 2.45 GHz in frequency sent through a wave guide pipe 18 from an external microwave power source 16 pass to be introduced into the plasma formation chamber 10. Additionally stated, the illustration of matching device, a microwave power meter, an isolator, etc. as usually provided between the wave guide pipe 18 and the microwave introducing window 14 is omitted in FIG. 1.
A plasma extracting means 20 is provided on the border between the plasma formation chamber 10 and the object treatment chamber 12. The plasma extracting means 20 is usually constituted of a quartz ring, the central aperture of which serves as a plasma extracting aperture 22, through which a plasma is extracted from the plasma formation chamber 10 into the object treatment chamber 12. If necessary, the plasma extracting aperture 22 may be provided with a grid (grid electrode) (not shown in FIG. 1).
The object treatment chamber 12 is provided with an object table 24 on which an object 28 to be treated, such as a wafer, a glass substrate or other substrate, is to be placed. The object table 24 is of such a construction that a DC or AC voltage can be applied thereto from the outside. The object 28 (substrate) is carried into the object treatment chamber 12 from the outside by means of a substrate transportation mechanism not shown in FIG. 1, and is placed and held on the object table 24 in such a way that magnetic circuit(s) 26. The impinging effect of plasma ions against the object 28 (substrate) works to effect a surface treatment, such as etching or film formation, of the object 28 placed on the object table 24.
FIG. 2 is a diagram showing the curves of relationship between the plasma extraction distance (distance between the plasma extracting means or panel 20 and the substrate 28) and the ionic current density, which were obtained using the foregoing conventional apparatus. In FIG. 2, the abscissa represents the plasma extraction distance (unit: mm), while the left and right ordinates represent the uniformity of the ionic current density distribution (.+-.%) in the surface of a substrate and the in-surface average value of ionic current density (mA/cm.sup.2), respectively. The substrate used is a wafer of 140 mm in diameter. As usual, Faraday cups were used at three points of the surface of the wafer to carry out the measurement. When I.sub.MAX and I.sub.MIN are the maximum and minimum ionic current densities, respectively, with I.sub.AVE being the average ionic current density, the uniformity of the ionic current density distribution in the surface of a substrate is calculated according to the formula: ##EQU1## The measurement was carried out under such conditions that N.sub.2 gas was introduced at a flow rate of 20 sccm into the plasma formation chamber 10 under an internal gas pressure of 5.0.times.10.sup.-4 Torr, while at the same time introducing the surface thereof to be treated is confronted with the plasma extracting aperture 22.
A magnetic circuit(s) 26 constituted of, for example, a coreless solenoid coil(s) is provided around the outer periphery of the plasma formation chamber 10. A magnetic field is formed inside the plasma formation chamber 10 by means of the magnetic circuit(s) 26.
Further, this conventional apparatus is provided with a gas introducing system 30 for introducing a gas into the plasma formation chamber 10 in the case where the apparatus is used to carry out an etching treatment, or with gas introducing systems 30 and 32 for introducing respective gases into the plasma formation chamber 10 and the object treatment chamber 12, respectively, in the case where the apparatus is used to carry out a film deposition or formation treatment. The kind(s) of gas(es) to be introduced is chosen depending on the type of etching or the kind of film to be deposited or formed. A vacuum pumping system 34 is constituted of, for example, a turbo-molecular pump and a mechanical pump. In this conventional apparatus, microwaves are introduced into the plasma formation chamber 10 wherein a magnetic field with a predetermined intensity is formed by means of the magnetic circuit(s) 26, and induces therein electron cyclotron resonance (ECR), through which an energy is generated to form therewith a plasma from a plasma-forming gas introduced into the plasma formation chamber 10. The plasma formed in the plasma formation chamber 10 is extracted, in the form of a plasma stream, into the object treatment chamber 12 with the aid of the action of a divergent magnetic field formed with the thereinto microwaves generated at a microwave power of 600 W. The ionic current density distribution in the surface of the substrate and the in-surface average value of ionic current density, measured using the above-mentioned conventional apparatus under the foregoing conditions, are shown in FIG. 2 with plotting, wherein the Curve I refers to the uniformity of the in-surface ionic current density distribution while the Curve II refers to the in-surface average value of ionic current density. According to these experimental data, the larger the plasma extraction distance, the better the uniformity of the in-surface ionic current density distribution, but the lower the in-surface average value of ionic current density.
FIG. 3 shows the curves of relationship between the ionic current density in the surface of a substrate and the distance from the center of the substrate at respective applied microwave powers of 600 W and 800 W.
In FIG. 3, the abscissa represents the distance (mm) from the center of the substrate, while the ordinate represents the ionic current density (mA/cm.sup.2). The experiment was carried out under substantially the same conditions as in the experiment relating to FIG. 2 except for the applied microwave power. In this experiment, the plasma extraction distance was set to be 60 mm. As will be understandable from the experimental data included in the Curve III (at 600 W) and the Curve IV (at 800 W), the value of ionic current density was highest at the center of the substrate with a tendency to lower toward the periphery of the substrate. Further, when the microwave power was larger, the difference in the value of ionic current density was larger between the center of the substrate and the periphery of the substrate than it was when the microwave power was smaller. The reason for this is believed to be that a plasma is concentrated in the vicinity of the central axis of the plasma formation chamber when the microwave power is increased, and that the concentrated plasma is guided from the inside of the plasma formation chamber into the object treatment chamber with the aid of the action of a divergent magnetic field formed with the solenoid coil(s) 26, with the result that the concentrated plasma reaches, as such, the substrate to provide the highest plasma density in the center of the substrate.
Since the uniformity of treatment speed distribution is determined by the distribution of plasma density and, in other words, ionic current density, such a large difference in the value of ionic current density between the center of a substrate and the periphery of the substrate as provided in the case of foregoing conventional apparatus results in a failure to provide a uniform treatment speed over the whole area of the surface of the substrate.
Thus, the use of the foregoing conventional apparatus of system cannot provide a uniform treatment of a substrate over the whole surface thereof. If the treatment of a substrate according to the above-mentioned conventional system is to be effected even a little more uniformly over the whole surface of the substrate, the plasma stream extraction distance must be set to be larger, with the disadvantageous result that the size of the apparatus is inevitably increased. Further, as the plasma extraction distance is increased, the ionic current density is lowered to disadvantageously reduce the treatment speed. Furthermore, enlargement of a microwave power source is necessary in order to increase the treatment speed.
An object of the present invention is to provide a small-sized microwave plasma treatment apparatus with which uniform etching as well as uniform film deposition or formation can be effected in various cases including the case where a large-diameter substrate is surface-treated and the case where the applied microwave power is increased.
In accordance with the present invention, there is provided a microwave plasma treatment apparatus comprising:
a cylindrical plasma formation chamber,
an object treatment chamber adjoining the plasma formation chamber in the direction of the central axis thereof,
a magnetic circuit(s) provided around the periphery of the plasma formation chamber,
a plasma extracting means provided on the border between the plasma formation chamber and the object treatment chamber,
an even-numbered plurality of auxiliary magnets numbering at least two and provided around the periphery of the plasma formation chamber and on the inner side of the magnetic circuit(s), and
a microwave introducing means provided along the central axis of the plasma formation chamber and on the opposite side thereof to the object treatment chamber;
wherein the plasma formation chamber is provided with a plasma-forming gas introducing port,
wherein the object treatment chamber is provided with an object table for placing thereon an object to be treated,
wherein the auxiliary magnets are arranged symmetrically with respect to the central axis of the plasma formation chamber in such a way that the magnetic poles of every auxiliary magnet are respectively reverse in polarity to the adjoining magnetic poles of an auxiliary magnet(s) adjacent thereto, and
wherein the electron cyclotron resonance phenomenon induced by an electric field formed by microwaves introduced into the plasma formation chamber through the microwave introducing means and a magnetic field formed by means of the magnetic circuit(s) and the auxiliary magnets is utilized to turn a plasma-forming gas introduced into the plasma formation chamber into a plasma, which is then extracted into the object treatment chamber through the plasma extracting means to irradiate therewith the object placed on the object table.
In the apparatus of the present invention, it is preferable that the above-mentioned auxiliary magnets be arranged in one or more groups in the direction of the central axis of the above-mentioned plasma formation chamber in such a way that the magnetic poles of each of auxiliary magnets in one group are reverse in polarity to the adjoining magnetic poles of an auxiliary magnet(s) in other group(s) adjacent thereto, if any.
In the apparatus of the present invention, it is preferable that the inner wall of the plasma formation chamber or at least a plasma introducing window which serves as part of the inner wall of the plasma formation chamber be made of an insulating material, and that microwaves are introduced into the plasma formation chamber through the above-mentioned inner wall or the plasma introducing window.
In the apparatus of the present invention, it is preferable that the above-mentioned microwave introducing means have an inner wall surface extending from the outside of the plasma formation chamber to the inside of the plasma formation chamber, disposed symmetrically with respect to the central axis of the plasma formation chamber and providing the inner diameter increasing along the above-mentioned central axis toward the above-mentioned extracting means.
In the apparatus of the present invention, it is preferable that the portion of the microwave introducing means located on the outside of the plasma formation chamber comprise a wave guide pipe, while the portion thereof located on the inside of the plasma formation chamber be constituted of a block made of a dielectric material.
In the apparatus of the present invention, it is preferable that the plasma formation chamber be shaped in the form of a cup opened on the side of the plasma extracting means, and that the microwave introducing means be shaped in the form of a horn opened on the side of the plasma extracting means and put on the plasma formation chamber to cover the same.
In the apparatus of the present invention, it is preferable that the auxiliary magnets be each constituted of a permanent magnet.
As described above, the microwave plasma treatment apparatus of the present invention is of such a construction that a plurality of the auxiliary magnets are arranged around the periphery of the plasma formation chamber not only in such a way that a plurality of auxiliary magnets are disposed along the circumference of the plasma formation chamber while a plurality of auxiliary magnets may also be arranged in the axial direction of the plasma formation chamber, but also in such a way that the magnetic poles of every auxiliary magnet are respectively reverse in polarity to the adjoining magnetic poles of an auxiliary magnet(s) disposed adjacent thereto either along the circumference of the plasma formation chamber or in the axial direction thereof.
The foregoing arrangement of the auxiliary magnets around the periphery of the plasma formation chamber enables a strong magnetic field to be formed in the neighborhood of the inner wall surface of the plasma formation chamber to thereby form a high-density plasma in the above-mentioned neighborhood. This enables the whole high-density plasma to be substantially uniform throughout the inside of the plasma formation chamber.
Where the apparatus of the present invention is of such a construction that the aforementioned microwave introducing means has an inner wall surface extending from the outside of the plasma formation chamber to the inside of the plasma formation chamber, disposed symmetrically with respect to the central axis of the plasma formation chamber and providing the inner diameter increasing along the above-mentioned central axis toward the aforementioned plasma extracting means, an electric field formed by microwave can be introduced into the plasma formation chamber without any loss thereof to thereby uniformize the plasma density, the uniformity of which can be further enhanced by virtue of the shape of the inner wall surface of the plasma formation chamber which is wholly or partially shaped in a top-cut conical form.
In the present invention, the effect of uniformizing the plasma density can be further enhanced where the wall of the plasma formation chamber is made of a dielectric material and the structure thereof is such that microwaves can be introduced into the plasma formation chamber uniformly throughout the inside of the plasma formation chamber.